Dragon Quest Swords: The Masked Queen and the Tower of Mirrors

first time you defeat Xiphos

he drop one

best advice to give you here try to have Ice Age master stroke it's more easy with it

defeat Valgirt on S grade and you'll have 2 and 1 Yggdrasil dew

it's mirror bosse advalable only after defeating Xiphos

slash like crazy on each dragon head close to 200 hits or over

block as many as possible attack and you'll get an easy S grade

For me, it was easiest to get orichalcum against the mirror boss, Der Gib (Big Red). Kylohk's walkthrough (www.gamefaqs.com/console/wii/file/932979/55684) provides excellent instructions for quickly and easily beating him.
Getting S-Rank nets you one orichalcum.

The first time you defeat Xiphos, you recieve one Orichalcum lump. You can also recieve 2 Orichalcum lumps for deafeating Valgrit in the Olde Reflectory and recieving an S ranking.

I don't know of any other sources of Orichalcum.